Our efforts to understand the role of SOCS family molecules has been focused on four members, namely SOCS1, SOCS3, SOCS4 and Cish. SOCS1 is highly expressed in immature thymocytes and is necessary to suppress cytokine signaling in pre-selection thymocytes. We have recently demonstrated that the transcription factor ThPOK induces expression of SOCS1 and that the CD4/CD8 lineage choice in the thymus is controlled by ThPOK-dependent pathway of SOCS molecule expression. We generated a series of ThPOK transgenic mice with graded expression of transgenic ThPOK, and we assessed CD4/CD8 lineage choice and T cell development in the thymus of these mice, in the presence or absence of SOCS1. Currently, we are expanding this study and we are assessing the effect of ThPOK overexpression on mature CD4 cells to address a continued role of ThPOK in SOCS molecule expression in peripheral T cells. Specifically, we are addressing the role of ThPOK in CD4 effector T cell differentiation. On the role of SOCS3 in T cells, we generated and characterized SOCS3 transgenic mice where the transgene is driven by a human CD2 mini-cassette. Overall T cell development in SOCS3-transgenic mice was comparable to wildtype mice, except for a selective loss (about 50% reduction) of CD8SP thymocytes and peripheral CD8 T cells. These data are in agreement with a cytokine requirement for CD8 lineage commitment and homeostasis, and they demonstrate that SOCS3 also plays a role in CD4/CD8 lineage choice. To directly test the effect of SOCS3 on cytokine receptor signaling, we examined IL-4, IL-6, IL-7 and IFN-g signaling in T cells by assessing downstream STAT6, STAT3, STAT5 and STAT1 activation, respectively. Interestingly, we found a broad inhibitory effect of SOCS3 on all tested cytokines, which suggested that SOCS3 interferes with cytokine receptor signaling utilizing a mechanism that possibly involved direct inhibition of JAK molecules. Experiments to understanding the molecular pathway of SOCS-mediated cytokine suppression are currently under the progress. In contrast to SOCS1 and SOCS3, we found that Cish is expressed only at low levels in thymocytes and in resting T cells. However, we found that TCR stimulation induces expression of Cish which is not the case for SOCS1 and SOCS3. These results suggest distinct roles for Cish and SOCS1/SOCS3. In T cells, Cish has been shown to inhibit STAT5 phosphorylation by gc cytokines. However, why Cish expression is induced by TCR signaling and not by cytokine signaling is unclear. To further assess the role for Cish, we have generated Cish transgenic mice by expressing a FLAG-tagged Cish cDNA under the control of the human CD2 mini-cassette. We are currently analyzing T cell development and function in Cish transgenic mice. Finally, we identified SOCS4 as a SOCS family member that is highly expressed in immature thymocytes. To address its role in T cell development, we generated a T cell-specific SOCS4 transgenic mouse and found that SOCS4 overexpression suppresses T cell development and activation. Interestingly, we also found that constitutive expression of SOCS4 impaired peripheral T cell survival and homeostasis so that naive T cell numbers were significantly reduced and apoptotic activity was markedly increased. We are currently interrogating the specific downstream effects of SOCS4 overexpression to further understand how SOCS4 interferes with T cell development and differentiation.